Here's my summary of the mass damper affair:
Renault first introduced the mass damper around September last year, so only had it for the last few races of the 2005 championship. The FIA's technical delegate was quite happy with it then and made no moves to outlaw it for 2006. The rules governing movable parts which have influence on the aero have not significantly changed from last year either.
The FIA now say they have 'evidence' that the principle purpose of the systems (as currently developed) is to influence aero performance, and as the damper moves and is not rigidly fixed to the sprung part of the car, the FIA have moved to ban the mass damper. The FIA will not divulge the nature or the source of the evidence they based their ruling on.
At the German GP Renault presented the test car for scrutineering with the damper still fitted. They then introduced evidence that showed the principle benefit of the mass damper was to improve mechanical grip by reducing variation in the tyre's contact patch. They also showed data indicating the influence on aero performance was negligible, with negligible change in mean aero load and negligible change in aero load variation. The stewards, who apparently do not have access to the 'evidence' that the FIA claim to have, declared the mass damper legal. The FIA promptly appealed against the Steward's decision, and the appeal will be heard late August/early September.
Renault, although cleared to run the mass damper in Germany, chose not to, in part due to the risk of having results disqualified if the FIA's appeal of the steward's decision is successful. Renault have now got an assurance from the FIA that even if the FIA win the appeal and the mass damper is again declared illegal, that they will ask the court not to penalise teams who choose to run it in the meantime. Renault plan to use the mass damper in Hungary.
The mass damper is estimated to be approx 10Kg (and so is not a huge 'free weight' in the event of accidents) and was stated to have been used by 7 teams during 2006. This is rumoured to include Ferrari and Red Bull. The benefit of the mass damper is considered to be greater for Michelin shod teams due to the Brdgestone tyres having stiffer sidewall construction.
Conspiracy stories aside, Ferrari's Jean Todt has recently stated that Ferrari believe the system to be illegal. This is despite running the system themselves until the ban prior to the German GP. Ross Brawn, when asked if their system was as good as Renault's, said "I don't know how you judge that. It's a useful system for us. I don't know how useful it is for them. We've used it all year, but we're not using it today."
For me, the FIA's ruling is quite a large stretch, and the timing stinks. It is hard to not think it's meddling to spice up a championship. There are plenty of FIA 'pro-Ferrari' and 'anti-Michelin' conspiracies doing the rounds, but for me, without access to the FIA's confidential 'evidence', their ruling is laughable, especially as nearly every component of the car's suspension system could be outlawed for the same reasons. Also, the timing, just before a summer testing ban and in the last third of Michelin's last F1 season (where they probably don't want to throw money at tyre development) doesn't help dispel the conspiracy theories. The FIA's evidence, which must surely be presented at the appeal for it to have any credibility, will need to be rock solid, and not just be Ferrari produced data.